1. Field of the Invention
The present invention relates to a touch probe, used on a coordinate positioning machine to determine contact between a stylus and a surface. In particular, the invention relates to an electro-mechanical contact used in such a probe, and to materials used to form such a contact.
2. Discussion of the Related Art
Existing known touch probes provide a fixed structure by which the probe is supported on the movable arm of the machine on which the probe is to be used, and a stylus supporting member supported on the fixed structure at a number of locations in a mechanically repeatable rest position, out of which the stylus-supporting member may be deflected when a deflecting force is applied thereto, and to which it may return when the force is removed. At each of these locations, a surface provided on the stylus-supporting member abuts a corresponding surface provided on the fixed structure. Contact between a stylus carried b the stylus-supporting member and a surface whose position is to be measured causes movement of the stylus-supporting member out of the rest position. Such contact may thus be determined by examining the electrical resistance of the contacts between corresponding surfaces on the fixed structure and stylus-supporting member. These contacts are known as electro-mechanical contacts. Probe which employ electro-mechanical contacts are known, and are described for example in U.S. Pat. No. 4,153,998.
The basic requirements for an electro-mechanical contact in such probes are best illustrated with reference to FIG. 1 of the accompanying drawings, which shows a conducting sphere 10 resting on a conducting plane surface 12, and an electrical circuit 14 of which the sphere 10 and the plane surface 12 form a part. One requirement of an electro-mechanical contact is that the sphere 10 may be removed and replaced repeatedly upon the surface 12, and that both before and after a cycle of removing and replacing the sphere 10 on the surface 12, the center O of the sphere 10 should lie at a distance h above the surface 12. This requirement is known as mechanical repeatability i.e. that the mechanical relationship between the sphere 10 and the surface 12 is repeatable (in respect of the distance h) over a large number of cycles of breaking and re-seating of the mechanical contact. Another requirement of an electro-mechanical contact relates to the changes in electrical resistance which occur in the circuit 14 as a result of breaking and re-seating of the contact. More specifically, it is a requirement that the characteristic changes in resistance which occur during a breaking and re-seating cycle are the same over a large number of cycles, such that during breaking of the contact, a given value of resistance repeatedly corresponds to a given distance of the center O of the sphere 10 from the surface 12. This is known as electrical repeatability.
In the above described probe, electronic detection circuitry associated with the probe emits a trigger signal when the resistance of a pair of contacting surfaces reaches a pre-determined threshold (the probe is thus sometimes known as a touch-trigger probe). Emission of this trigger signal indicates contact between a stylus supported by the probe and a workpiece whose surface is to be measured. (N. B. the signal is sent to the control of the machine, and is used by the machine to determine the position of the movable arm and thus the position of the surface). Similarly, a reduction in the value of resistance of the contact to a resistance value below the pre-determined threshold indicates that the stylus supporting member has returned to its rest position with respect to the fixed structure (which is essential for a probe of this type).
The choice of a material for an electro-mechanical contact in a touch trigger probe has in the past been governed by the perceived requirements for the material to have the maximum possible hardness with a smooth surface finish, while simultaneously being a conducting material. The hardness and surface finish requirements were chosen to provide good mechanical repeatability of the stylus-supporting member (the smooth surface finish aiding this by reducing friction), and resistance to physical damage of the contacts, thus prolonging the useful life of the probe. Known such touch-trigger probes employ tungsten carbide having a cobalt binder. In order to prevent oxidation of the contacts during operation, the contacts are coated in oil.
A long standing problem with known touch-trigger probes, which occurs after a relatively large number of unseating and re-seating cycles of the stylus supporting member with respect to the fixed structure, is that the resistance of one or more of the contacts fails to drop below the predetermined threshold which signals re-seating of the stylus supporting member in the fixed structure. This phenomenon is known as "re-seat failure".
It has been postulated that this failure to re-seat is a mechanical phenomenon due to friction at one or more of the contacts, acting to prevent re-seating of one or more of the other contacts. In an effort to overcome this perceived problem, methods of operating the machine on which the probe in question was used have been proposed which effectively "tap" the stylus supporting member to attempt to cause it to return to its rest position (see e.g. GB 2070249 and U.S. 4815214). A further mechanical reason advanced for re-seat failure suggested that a small scale mechanical shift between the contact surfaces, from one cycle of unseating and re-seating to another, led to a change in the material providing the contact area between the contact surfaces. For example such a mechanical "shift" might result in the material of the contact area of one of the surfaces changing from being provided by the binder material of the tungsten carbide, to a "carbide particle", with a consequential change in resistance (see for example GB 2145523).
GB 2145523 also postulates that a re-seat failure is sometimes the result of oxidation layers forming over the tungsten carbide contacts during the operating life of the probe.
The disclosure of GB 2145523 proposes, as a solution to both perceived problems (i.e. mechanical shift of the contact surfaces and generation of an oxidation layer), the coating of the contact surfaces with a coating of titanium carbide or titanium nitride. Such coatings were thought to provide improved mechanical repeatability due to the higher hardness of the coating material, while the electrical resistance characteristics of the contacts were thought to be improved by an apparent homogeneity of the coating material.
None of the above prior art solutions have been found to be satisfactory. Methods providing mechanical disturbance to the contacts, while they overcome some individual instances of re-seat failure, have not provided a solution which prevents such a re-seat failure in the first place. Attempts to cure the perceived problem of oxidation by coating the contact surfaces with oil have also failed to solve the problem of the re-seat failure. We have found titanium nitride or titanium carbide coatings to be unsatisfactory because they provide poor mechanical repeatability, the resistivity of the coating is too high, and the coatings have a tendency to flake off during operation of the probe.